The present invention relates, in general, to optical parametric oscillators, and more particularly to such an oscillator incorporating a urea crystal as the nonlinear medium.
The very high power densities made available by lasers have made it possible to observe the nonlinear effects of crystals, including the frequency doubling effect. This occurs when a radiation of frequency .nu., on propagating through some crystalline materials, emerges as radiation consisting of a mixture of two frequencies, the original frequency .nu. and a new frequency 2.nu., the double frequency component having a wavelength which is one-half the incident radiation.
The explanation of such nonlinear effects lies in the way in which a beam of light propagates through a solid material, the electromagnetic radiation interacting with dipoles in the material and causing them to oscillate. As the intensity of the radiation increases, so does the amplitude of vibration and eventually harmonics are produced. The strongest harmonic is the second, at twice the frequency of the incident radiation, and for this reason the frequency doubling effect is also referred to as second harmonic generation. However, not all materials exhibit frequency doubling.
Early experiments in frequency doubling gave very low conversion efficiencies of about 1%, due to the fact that dispersion within the crystal caused the frequency doubled light to travel at a different velocity than the incident light. This resulted in destructive interference and periodic fluctuations in the intensity of the frequency doubled light. However, it was found that equalization of the velocities of the light, or phase matching, could be achieved using birefringent crystals such as ADP or KDP, if the light dispersion is less than the birefringence. Such materials provide 20-30% efficiencies, but are limited in the frequencies of light that can be produced. Other materials have been tried, but optical damage often limits their operation.
Frequency doubling is a specific example of what is known as the sum-frequency generation process. Consider two sinusoidally varying electromagnetic fields of different frequencies denoted by E.omega..sub.1 and E.omega..sub.2 and with frequencies .omega..sub.1 and .omega..sub.2 respectively, then the polarization produced by these fields acting together can be expressed in the form: EQU P=.chi.E.omega..sub.1 E.omega..sub.2 ( 1)
where .chi. is the susceptibility. It can easily be deduced that electromagnetic waves at two new frequencies .omega..sub.3, .omega..sub.3 ' will be produced and these frequencies are given by EQU .omega..sub.3 =.omega..sub.1 +.omega..sub.2 ( 2) EQU .omega..sub.3 '=.omega..sub.1 -.omega..sub.2 ( 3)
Such effects only occur in non-centrosymmetric materials. In the case of frequency doubling E.omega..sub.1 and E.omega..sub.2 are identical and equations (2) and (3) hold to give a new wave of frequency 2.omega. and a d.c. component respectively.
E.omega..sub.1 and E.omega..sub.2 are often known as the pump and the signal, the pump being of higher frequency.
It can be seen from equations (2) and (3) that if .omega..sub.1 and .omega..sub.2 are fixed, then .omega..sub.3 is also fixed as a sum or difference frequency. However, if only one of the three frequencies is fixed then the other two frequencies .omega..sub.1 and .omega..sub.2, are free to range over many values, provided the sum of their frequencies is equal to that of the fixed frequency.
The inverse of the foregoing sum-frequency process is the optical parametric process, where two variable frequencies are decided by the particular phase matching used. Only one pair of frequencies can be phase matched at a time. By adjusting the phase matching parameters, e.g. the temperature or orientation of the nonliner crystal, the laser can be tuned over a range of frequencies. The process of converting a wave having a frequency .omega..sub.3 into two lower frequency waves .omega..sub.1 and .omega..sub.2 is called the optical parametric process.
The parametric oscillator uses a nonlinear crystal to convert the pump light into two signals, the sum of whose frequencies equals the frequency of the pump light. Thus, the oscillator produces two outputs, the signal, and a difference frequency, which is referred to as the idler. These frequencies can be tuned by index matching, as by rotating the crystal or by controlling the temperature of the crystal. Since a parametric oscillator has gain at its signal and idler output frequencies, it can be used as an amplifier at either frequency.
Tunability of a parametric amplifier or oscillator is important because there are optical instruments whose performance would benefit tremendously by replacing conventional light with a tunable laser. Absorption spectrometers, for example, employ a white light source and narrow regions of the spectrum are selected by a narrow band filter which usually takes the form of a diffraction grating. The amount of light obtained at any one wavelength is clearly only a very small fraction of the total light energy emitted by the lamp. The use of a laser would enable all of the energy to be concentrated into any required region of the spectrum. Further, the narrow linewidth of the laser would result in much higher resolution. Flash photolysis, in which fast chemical reactions and short lived chemical compounds can be examined, would also clearly benefit from a pulsed tunable laser.